Len Brown must be kicking himself. Three weeks ago, a Herald-Digipoll survey showed the Manukau City mayor holding an 11.4 percentage point lead over his rival for the Super City mayoralty, Auckland City's John Banks.

Today, Mr Brown will be wondering how seriously he has been tripped up by, as is so often the case, a relatively trivial sum of money.

Having admitted making personal purchases on his council credit card, including a $148 mini hi-fi system, he says that he has now reimbursed the council for personal expenditure totalling $579.27.

The episode leaves problems for Mr Brown. First, he has allowed himself to become the subject of questioning at the very time that the issue of politicians' expenses and allowances has been under intense scrutiny in New Zealand and in Britain.

This, at the very least, smacks of carelessness, while also suggesting a lack of judgment. Mr Brown may well have thought that no harm would be done by charging the hi-fi to his council credit card because he did not have his personal card and it was "essential" to get it that day.

Yet he should have been aware of the perception if his use of the card in this manner became public.

It can hardly have passed him by that in February, Phil Heatley resigned from his housing and fishing portfolios after using his ministerial credit card outside the rules.

Mr Brown says he tries to "set the best examples on all fronts", and his credit-card spending is approved by the council's finance director and audited every six months. The council chief executive says the mayor has not contravened any rules.

Yet Mr Brown must have known that using the council card for personal spending fell somewhat short of the ideal. An "administrative mistake" may, indeed, have led to a family event, a Christmas party at Auckland's Rendezvous Hotel, being charged to that card, rather than his personal one.

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But that does not explain his use of the card to pay a $115 grocery bill. At a time when politicians at every level know they must face a greater level of disclosure, he should have been intent on avoiding any hint of embarrassment. Quite simply, no personal spending should have been charged to a credit card paid for by the ratepayers of Manukau City.

Mr Brown's indiscretion has provided welcome ammunition for his political rivals. This is doubly so because Mr Banks has been able to claim that he never had a credit card as mayor or when he was a National cabinet minister.

"I just think they are such high risk and it leaves people open to temptation and abuse," he said. Nor, said Mr Banks, as a matter of emphasis, would he have a credit card if he became the first Super City mayor.

Adding to Mr Brown's discomfort, the Mayor of Hamilton, Bob Simcock, was quoted as saying that he also did not have a council credit card. He paid for official expenses from his own pocket and was reimbursed after vetting by the council chief executive.

There is still quite some time until the Super City election on October 9. Mr Brown's pledge to bring people together, rather than divide them, has clearly struck a chord. But this episode points to his relative inexperience, especially when he is cast alongside Mr Banks.

Last night, Mr Brown made the logical next move, seeking to regain lost ground by chopping up the credit card on national television.

But there is absolutely no need for such cards. They serve no purpose, and hint at an inflated view of entitlement. At least putting it on the ratepayer, even if only as a matter of short-term convenience, will no longer be a temptation now Mr Brown's council card has been scissored.